The IEEE 802.5 Token Ring Local Area Network uses a balanced twisted pair wire for data transmission. In the system data is transmitted using an encoding technique known as differential Manchester encoding. As the signal travels down the twisted pair, it undergoes phase and amplitude distortion which varies with the length of the transmission line and the data rate. This distortion gives rise to intersymbol interference and may be corrected by using an equalizer circuit located at the receiving end. FIG. 1 illustrates a prior art equalization circuit suitable where a single data rate is employed. The equalizer has a pair of input terminals 11 and a pair of output terminals 12. A differential amplifier/filter including a pair of transistors 14 and 15 provides an output at the pair of terminals 12 which is a function of the differential input applied at the pair of input terminals 11. A pair of constant current sources 16 and 17 regulate the total current through transistors 14 and 15. The frequency response and group delay characteristics for small signals can be tailored by selection of a suitable impedance Z. (Voltage transfer from input to output=2*RL/Z) As the input signals gets larger, corresponding to shorter lengths of transmission line, the transfer function from input to output becomes non-linear and reduces the amount of frequency and group delay correction. When the input signal at the pair of input terminals 11 is sufficiently large, the collector current through transistor 14 is cut off. Current flowing through transistor 15 will flow through impedance 18 and current source 16 as well as current source 17. Conversely, when the collector current through transistor 15 is cut off, current flowing through transistor 14 will flow through impedance 18 and current source 17 as well as through current source 16. This circuit is inserted in the data path at the receiver.
If the transmission media, that is the twisted pair used to carry signals between stations, is to carry signals having different data rates at diffferent times, the equalizer illustrated in FIG. 1 cannot cope with different data rates such as 4 or 16 megabits. A pair of equalizers each suitable for equalizing the line for the different data rates will be required. FIGS. 2 and 3 show early attempts at solving the problem by providing two equalization circuits. In FIG. 2 the signal from the input is applied to a first equalizer 1 and a second equalizer 2 and an electronic analog switch is used to select one or the other equalizer. In the equalizer of FIG. 3, two impedances Z.sub.1 and Z.sub.2 are selected depending upon the data rate of the signal applied to the transmission media. Z.sub.1 is selected, for example, to compensate the transmission media for a 4 megabit data rate while Z.sub.2 will be selected to compensate the transmission media for a signal at a 16 megabit rate.
The circuits illustrated om FIG. 2 and FIG. 3 are potential solutions to the equalization of multi-data rate transmission systems. However, for reasons which will be explained below, neither of these circuits is desirable.
The obvious disadvantage of the circuit illustrated in FIG. 2 is the duplication of components and the additional space required for providing multiple equalizers. A second disadvantage is the possibility of feedthrough of unwanted signal from the deselected equalizer. The extent of the feedthrough would depend entirely on the type of switch used. If a relay is used, the open contacts must have very little capacitance across them. Capacitance as small as 1 pico-farad could allow excessive feedthrough of unwanted signals. This low value of capacitance is not difficult to achive with relay contacts. However, relay contacts have other disadvantages such as requiring power for operating the coil and requiring a large amount of area on circuit boards, etc. Analog switches used as an alternative have unique problems of their own. While they solve the space problem, they have electrical characteristics which are undesirable. They have a relatively large series resistance (50-100 ohms) for closed switches and large capacitances (substantially greater that 1 pico-farad) for open switches. These characteristics add undersirable signal distortion and allow excessive feedthrough of unwanted signals. In additon, there is excessive capacitive coupling between different switches fabricated on a single chip.